Psychopharmacology - in vivo neurochemistry and pharmacology

Psychopharmacology - in vivo neurochemistryand pharmacology

Paul Grasby, Andrea Malizia and Chris BenchMRC Cyclotron Unit, Clinical Sciences Centre, Hammersmith Hospital, London, UK

Positron emission tomography (PET) and single photon emission computerizedtomography (SPECT) brain imaging techniques are powerful tools for theinvestigation of human neurochemistry and psychopharmacology.These versatilein vivo methods can be used to measure brain receptor populations in psychiatricillness, to determine receptor occupancy and tissue pharmacokinetics ofpsychotropic drugs and to examine the functional effects of drug-inducedneurotransmitter manipulation.This chapter reviews recent innovative basic andclinical studies, of relevance to psychiatry, and discusses methodologicaldevelopments that extend the use of these techniques.

The direct assay, monitoring and measurement of central neurochemicalsystems in vivo in man is at present limited to radioisotope imaging andmagnetic resonance spectroscopy. This chapter discusses positronemission tomography (PET) and single photon emission computerizedtomography (SPECT) derived measures of regional neurochemistry,pharmacology and physiology of relevance to psychiatry. Although onlya fraction of the some 100 neurochemicals and 300 receptor subtypesidentified in the human brain can be imaged or measured in vivo, theprocesses that are measurable, namely monoaminergic, cholinergic,opioid and benzodiazepine receptors, regional cerebral blood flow, andglucose and oxygen metabolism, are of considerable biological relevance.

Imaging neurochemistry and pharmacologywith radioisotopes

PET and SPECT use radiolabelled molecules to measure physiologicalc d to- processes, biochemical pathways and receptor systems of interest1.

DrPaulGrasby, MRC Briefly, PET and SPECT involve: (a) production and incorporation of aCyclotron Unit, clinical radioisotope into a molecule of biological interest to form a radiotracer

Sciences Centre, m a t j s administered to man; (b) imaging of the distribution of theDuCbne Road London radiotracer over time in living human brain using PEI or SPEC 1 cameras

W12 0HS, UK. to detect the emitted gamma radiation from the decaying radioisotope;

Britith Mm£cal Bulletin 1996^2 (No. 3):513-526 ©Ttw Briti.h Council 1996

Downloaded from https://academic.oup.com/bmb/article-abstract/52/3/513/287921by gueston 13 April 2018

Biological psychiatry

and (c) quantification of a physiological or pharmacological parameter ofinterest, such as receptor number or regional cerebral blood flow, fromthe analysis of the time course of radioactivity in the brain. The relativemerits of PET and SPECT techniques remain hotly debated. Absolutequantification is less of a distinguishing feature than previously, howeverPET continues to have a distinct advantage in terms of sensitivity, spatialresolution, and dynamic and repeated measures. However, SPECT is lessexpensive, more widely available and the previously restricted range ofSPECT radiotracers is expanding2. The most commonly used PETradioisotopes, produced by a cyclotron, are 15O, n C and 18F (with halflives of 2.03, 20.4 and 109.8 min, respectively) whilst modern SPECTradioisotopes include "mTc and 123I (with half lives of 6.02 and 13.2 h,respectively). State of the art PET and SPECT cameras have transaxialspatial resolutions in the realm of 4—5 mm and 6-8 mm, respectively (fullwidth half maximum) and can detect sub-nanomolar concentrations of

Table 1 Established and novel radiotracers of relevance to psychiatry

Radiotrac Target Application

PET radiotraceriC'3Ch/H213O

"F-FDG

"C-SCH 23390

"C-rodo pride

"C-FLB-457

"C-RTI121and55"F-fluorodopa

"C-flumazenil

"C-deprenyl

"C-carfentind

"C-dlprenorphine

''F-a ban serin

''F-setoperone1' C-m ethytspj pe ron a

"C-MDL-100907

SPECT radiotracer*T c123Modobenzamide

'"Upide prideral-QNBmH?crr

Blood flow

Glucose metabolism

Dopamine Di receptor

Dopamine D2 receptor

Dopamine D? receptor

Dopamine re-uptake site markers

Dopaminergk neurone density

Centra] benzodiazepine receptors

MAO-B odivity

Mu opioid receptor

Non-selective opioid receptors

5 - H T J receptor

5 - H T J receptor

Coriicd 5-HT2 receptors,

Striatal D2 receptors

5-HT2A receptors

Blood flowDopamine Direceptors

Dopamine Di receptors

Muscarink acetyfcholine receptors

Dopamine and 5-HT re-uptake sites

Successfully used to map brain areas involved in cognitive processing. Increasingly

applied to psyaSiatric populations32.

May be reploced by functional MRI techniques for bratn mapping.

Used for many resting slate studies in psychiatric populations, e.g. neuroleptic drug

effects23

Receptor occupancy studies with many neuroleptics". Recent report of decreased striakil

Di receptors in drug naive schizophrenics10.

Robust demonstration of no elevation of striatal D? receptors in drug naive

schizophrenics'. Striatal D2 receptor occupancy studies with many neuroloptics'.

High affinity Itgand; may enable extrastriatal D2 populations to be measured.

More suitable tracer for dopaminergic re-uptake site man ' 'C-nomifensine.

Racfiotrocer predominantly imageable in basal ganglia-cortical signal weak. Extensively

used in neurological studies. Some application to psychiatric studies41.

Labels all subtype* of central receptor. Occupancy, anxiety disorders/0 Icon di*™ studies

in progress.

Occupancy studios with novel MAO-B inhibitor. Studies in psychiatric groups in progress.

Occupancy studies with opiate receptor antagonist.

Studies in depressed patients in progress.

Being used in novel displacement studies17

Studies in depressed patients in progress.

Studies in depressed patients in progress,

Neuroleptic occupancy studies.

May be better bgand for imaging 5-KT2receptors than those above.

Many resting state and two scan activation studies in psychiatric patients.

Occupancy studies.

Pilot studies in progress.

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PETand psychopharmacology

receptors2. Current and novel radiotracers of interest for biologicalpsychiatry are summarised in Table 1.

Measurement of brain receptors in psychiatricpopulations

No major psychiatric illness has yet been shown to be conclusively adisease characterised by changes of brain receptors (i.e. changes of Bm x oraffinity). However, examining in vivo neurochemistry at the receptor levelis technically feasible, and hypotheses, can be often clearly guided byavailable post mortem and animal data. Indeed, PET/SPECT basedhypotheses should provide a compelling case for the in vivo assessment ofreceptor number as ex vivo autoradiographic techniques have consider-ably superior spatial resolution1. To date the most comprehensive studieshave estimated striatal dopamine D2 receptor number in schizophrenia.Following controversies over post mortem data indicating raised striataldopamine D2 receptors in patients with schizophrenia, PET/SPECTtechniques have proved to be the preferred approach to determinewhether raised dopamine D2 receptor number is biological marker of theillness or simply a result of chronic neuroleptic exposure. Wong andcolleagues, using 11C-N-methylspiperone as a radiotracer, reported 2-3-fold raised striatal D2 receptors in drug naive schizophrenics3. Since then,Farde4 and other investigators (see5), using n C raclopride, 123I-iodoben-zamide, 76Br-bromolisuride and also nC-N-methylspiperone, have failedto show such elevations of striatal dopamine D2 receptor number. Thesuggestion that these conflicting results arose from the different radio-tracer methodologies employed is not convincing since Farde's group hasused nC-N-methylspiperone, in addition to nC-raclopride, to show noelevation of D2 receptor number in schizophrenia6. Likewise, thesuggestion that nC-N-methylspiperone, binding to dopamine D2, D3

and D4 receptors, was detecting raised striatal D4 receptors whereasnC-raclopride, only binding to D2 and D3, was not7, appears questionablebecause the reported 6-fold increase of post mortem striatal D4 receptors(which formed the basis for the above suggestion) has not been conclusivelyreplicated8. In summary, total densities of the striatal dopamine D2

receptor do not appear to be raised in untreated schizophrenia. Thisnegative finding has focussed attention on the measurement of extra-striatal (cortical and limbic) dopamine D2 receptor numbers which areprobably more relevant to schizophrenia but which are in much lowerconcentration9. In this regard, the novel high affinity tracer nC-FLB 457appears promising as a marker of extrastriatal D2 receptors9. Striatal andextrastriatal dopamine Dj receptor number in schizophrenia is also beinginvestigated. Karlsson et al.w have recently reported a reduced concentra-

British Medical Bulletin 199d;52 (No. 3) 515



tion of striatal dopamine D t receptors in drug-naive schizophrenicpatients of the order of 20% using the radiotracer "C-SCH 23390,although this change has not generally been reported in post mortemmaterial9. This preliminary finding and the identification of dopamine D3,D4 and D5 receptor subtypes suggests that it remains worthwhileinvestigating dopaminergic receptor kinetics in schizophrenia in vivo9.

There have been fewer studies of central receptor populations indepressive and anxiety disorders5. For depressive illness, the focus hasbeen on the measurement of cortical 5-HT2receptor number. Using thePET radiotracers nC-N-methylspiperone, 18F-setoperone or 18F-altanser-in and the SPECT tracer 123I-ketanserin, studies have been hampered bythe either the lack of selectivity or the relatively low specific to non-specific signal obtained in the human brain11, making quantification ofcortical 5-HT2 receptor number in depression problematic. However, therecent development of nC-MDL 100907 promises to be a more usefultracer for studying 5-HT2A receptors12 and may help to resolve the statusof 5-HT2 receptors in depressive illness.

Another 5-HT receptor system of considerable interest in depressionand anxiety disorders has recently been delineated with PET. Using apotent, silent antagonist for the 5-HT1A receptor, WAY-100635, thedistribution of 5-HT1A receptors in the living brain have been mappedwith 11C-WAY-10063513. In accordance with autoradiographic studies,high levels of specific signal were seen in the hippocampal formation andmidbrain raphe nuclei (Fig. 1). The specific signal in the raphe nucleisuggests that somatodendritic autoreceptors as well as postsynapticpopulations of 5-HT1A receptors can be imaged and possibly quantified.

In anxiety disorders, decreased 123I-iomazenil (a central benzodiazepinesite marker) binding has been reported in panic disorder patients in thefrontal, temporal and occipital cortex14. This finding is tantalisingbecause of pharmacological evidence implicating a decreased sensitivityof the benzodiazepine receptor in patients with this disorder. However, itmust be viewed cautiously because of the control population studied(patients with epilepsy) and because the brain distribution of thisradiotracer, when studied early after injection, is particularly sensitive tobaseline blood flow changes. Thus this study needs to be replicated usingnormal controls and fully quantitative methods.

Measuring receptor occupancy, time course ofpsychotropic drug action and pharmacokinetic/pharmacodynamic relationships

If the receptor binding of a PET or SPECT radiotracer can be quantifiedas hmax or binding potential (BP), or expressed as a ratio of specific to

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PETand piychopharmacology

Fig. 1 The delineationof 5-HTj^ receptors in the

living human brain.Saggital (a), coronal (b)

and traniverse (c) imagesof "C-WAY-100635

binding in human brainco-registered onto a MRl

scan from the samesubject High levels of

labelling are seen in theraphe nudei (RN1,

hippocampal formation(Hi) and cingulate cortex

(GC) with low levels in thecerebellum (C) and visual

cortex (VC).

Fig. 2 Demonstration of dopaminergic modulation of a functional deficit in the anterior cingulate cortex in schizophrenia (a) Brainregions (yellow pixels) significant^ more activated (p < 0.05 corrected) during verbal fluency in normal subjects compared to drug-freeschizophrenics rendered onto an MRl image in stereotactic space. A relative failure of anterior cingulate activation is observed inschizophrenic patients, (b) Brain regions (yellow pixels) significantly more activated (p<0.05 corrected) during verbal fluency inschizophrenic patients compared to normals following injection of the dopaminergic agonist, apomorphine, rendered onto an MRl imagein stereotactic space. An activation of the anterior cingulate cortex (which is greater than that seen in normals^ b now observed inschizophrenic patients undertaking verbal fluency.

British Medical Bu//.hn 1996^2 (No. 3) 517



non-specific binding, the tissue kinetics of centrally acting drugs can beexamined. To date, most studies of this nature have been limited toinvestigating the occupancy of striatal dopamine Dj and D2 and corticalserotonin 5-HT2 receptors by antipsychotic drugs. Thus, using PET and"C-raclopride, Farde etal. have shown that clinically efficacious doses ofa variety of classical antipsychotics cause between 65-89% occupancy ofcentral dopamine D2 receptors15. This degree of occupancy occurs asrapidly as 2 h after acute administration of- the antipsychotic and acurvilinear relationship between antipsychotic dosage and D2 occupancyhas been described. Higher receptor occupancy (<85%) is associatedwith an increased incidence of extrapyramidal side effects (EPSE)16, thusthere appears to be a therapeutic window of occupancy of between 65 -85% which is antipsychotic and yet less likely to cause EPSE.

The threshold hypothesis for the dopamine D2 receptor (antipsychoticefficacy versus extrapyramidal side effects) is an important considerationin the preclinical and clinical evaluation of new antipsychotics andsuggests an exciting possibility for the use of receptor imaging techniquesin this process. The preclinical use of PET in dose finding studies forantipsychotics has recently been demonstrated in a dose ranging study ofthe novel antipsychotic CP-88,059-01 (Ziprasidone) using PET n C -raclopride binding17. In this study, subjects received a predose of between2-60 mg of Ziprasidone, orally, 5 h before PET scanning. Binding of n C -raclopride decreased in a dose dependent manner, and the lowest dose ofZiprasidone required for over 65% dopamine D2 receptor occupancywas 40 mg. At 60 mg predose, 85% occupancy was achieved. In a followup to this study, the time course of receptor occupancy after a single doseof 40 mg of Ziprasidone was established by scanning subjects between 4—36 h post dose. Such investigations of in vivo pharmacokinetics provideunique and invaluable information in guiding dosing regimes for phase IIand phase III clinical studies. Similar strategies could be used, forexample, in the development of new benzodiazepines using PET and n C -flumazenil, titrating occupancy against anxiolytic effect, or in theevaluation of selective MAO-A inhibitors using PET and nC-clorgyline,titrating level of enzyme blockade against antidepressant efficacy.

Using PET and nC-SCH23390, the Karolinska group have shown thattreatment with classical antipsychotics produces variable levels ofoccupancy of D ] receptors. Whereas haloperidol and sulpiride are notassociated with significant D^ occupancy, treatment with thioridazineand thioxanthenes such as flupenthixol results in between 16—44% Djoccupancy16. In comparison, efficacious doses of the atypical antipsy-chotic clozapine are associated with a relatively low D2 receptoroccupancy (38-63%) and a D! occupancy of 38-52%16. This unexpectedfinding of low D2 receptor occupancy, reproduced in different patientgroups with both PET and SPECT techniques, has challenged traditional

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PETand pjychopharmacology

theories of a necessarily simple relationship between D2 occupancy per seand clinical efficacy. Further evidence for this view comes from thosePET/SPECT studies showing that schizophrenic antipsychotic non-responders have the same levels of occupancy as responders18.

From the above it is clear that in vivo studies of neuroleptic binding tocentral receptors can aid the pharmacological understanding of the actionsof these compounds. PET is also proving useful in the characterisation ofatypical antipsychotics. For instance, the binding of antipsychotic drugs tocentral 5-HT2 receptors is a possible candidate for the mechanism of'atypicality' and 2 studies have demonstrated high cortical 5-HT2

occupancy with risperidone (80%) and clozapine (84-90%J19-20.Such labelling of brain receptor populations in vivo provides a unique

opportunity for determining the relationship between receptor kineticsand pharmacodynamic effects. In theory the application is only limited bythe availability of PET/SPECT and suitable radioligands for the receptorsof interest. A productive direction for understanding other psychotropicdrugs may come from studies of receptor occupancy versus behaviouraleffects including modulation of symptoms, side effects and response totreatment. The potential of this approach is illustrated by an elegant seriesof J ̂ -flumazenil experiments in non-human primates, where therelationship between occupancy at central benzodiazepine sites and druginduced seizure thresholds was measured. The proconvulsant effects of thebenzodiazepine receptor inverse agonists, Beta-CCM and DMCM, andthe anticonvulsant effects of diazepam, bretazenil and CL 218,872 weredirectly related to their occupancy at benzodiazepine sites; partial agonistsand inverse agonists had reduced intrinsic activity and high occupancy bycold (unlabelled) flumazenil (an antagonist) had no effect on the seizurethreshold21. Thus, when 50% of the central benzodiazepine sites wereoccupied, diazepam increased the seizure threshold by 180%, bretazenilby 20%, flumazenil had no effect and beta-CCM decreased it by 50%.

The further application of this methodology22 should provide uniqueinsights of CNS drug mechanisms in vivo, relating medicinal effects toreceptor occupancy. It is anticipated that this line of enquiry will be ofconsiderable interest in psychiatry. Furthermore, it could provideimportant insights in determining whether postulated changes in receptorsensitivity in disorders such as anxiety and depression are due to changesin receptor density or to postreceptor mechanisms.

Measuring the central effects of psychotropic drugs andthe neurotransmitter modulation of cognitive functions

In addition to measuring drug occupancy at central receptors in vivo,consideration must also be made of the cerebral regions and neuronal

BriHih Mtdkal BullmHn }996fi2 (No. 3) 519



circuits that are functionally targeted by administration of a drug. Atpresent, a convenient method for assessing the overall functional effect ofdrug-induced receptor activation is a PET or SPECT derived index ofneuronal activity, such as cerebral blood flow (rCBF) or glucosemetabolism (rCMRglu)23. Such drug effects can be examined underbrain resting or activation conditions. In both circumstances, drug-induced changes of rCBF or rCMRglu may reflect effects one or moresynapses away from the drugs initial pharmacological site of action24.Resting state studies of psychotropic drugs on rCBF or rCMRglu arecommon in comparison to combined drug and behavioural taskactivation studies. Many resting state studies have examined the effectof chronic neuroleptic medication on cerebral glucose metabolism inschizophrenic patients. Although the results have been somewhatvariable, the most consistent finding is that chronic medication, withmany, but not all, neuroleptics, increases striatal metabolism25. Althoughthe functional significance of this finding is somewhat contentious, someauthors have argued that a neuroleptic induced increase of striatalmetabolism (from a low baseline) is associated with response totreatment whilst unchanged striatal metabolism is associated with non-response25.

Regional CBF studies, unlike glucose metabolism measurements whichare limited to 1 or 2 scans per subject, are especially suited to activationstudies because of the potential for repeated measures with up to 12 scansof rCBF per subject. If a factorial design is adopted, a sophisticatedanalysis of drug effects can be made. Thus within a 6 run H2

15O PETrCBF study, the effect of a drug, the effect of a behavioural task and drug-task interactions has been examined26. Combined drug and behaviouralactivation designs have examined the neuromodulatory effects ofmanipulation of dopaminergic, serotonergic, and cholinergic neurotrans-mission on behavioural task induced increases of rCBF in volunteers andpatients. In one series of studies27-28, auditory-verbal memory testsprovided the behavioural tasks for groups of normal volunteers. Drugsused were apomorphine, a non-selective dopamine agonist, buspirone, a5-HT1A partial agonist and scopolamine, a muscarinic antagonist. Thechoice of drug was dictated by the neuromodulatory effects ofdopaminergic, serotonergic and cholinergic neurotransmission on cere-bral function29 and their implication in memory processes. Theexperimental hypothesis was that each drug, acting predominantly ondifferent neurotransmitter systems, would have regionally distinct effectson memory-task induced increases of rCBF. For example, given the roleof prefrontal dopamine neurotransmission in working memory tasks inmonkeys30, a specific prediction was that apomorphine, would modulatememory-induced rCBF activations maximally in the prefrontal cortex.Apomorphine indeed attenuated memory-induced increases of rCBF

520 Britith Mmdical Bulletin 1996^2 (No. 3)



maximally in the prefrontal region27. In contrast, buspirone attenuatedmemory-induced increases of rCBF maximally in the retrosplenial area ofthe posterior cingulate whilst scopolamine had more widespread effectsattenuating memory activations in the left and right prefrontal cortex andright anterior cingulate region28. The confirmation of a specific regionalinteraction between apomorphine induced manipulation of dopaminergicneurotransmission and memory function in the prefrontal cortex makesthe point that prefrontal function in man may be regulated bydopaminergic agents. This finding concurs with primate data whereprefrontal dopaminergic neurotransmission is pivotal for the execution ofmnemonic tasks and where signal-to-noise activity of prefrontal neuronesis modulated by microiontophoretic application of dopamine30.

These activation studies have been applied to patient populations. Inone of the first studies of this kind, Weinberger31 demonstrated aneuromodulatory role for dopamine on prefrontal function in schizo-phrenic patients. Prefrontal activations, measured as rCBF change duringthe Wisconsin Card Sort Test (WCST) were studied in a group ofmedicated schizophrenic patients treated with dextroamphetamine orplacebo. In the presence of dextroamphetamine, which releasesdopamine, the WCST activated the left dorsolateral prefrontal cortexcompared to a control task. In addition, dextroamphetamine wasassociated with a slight improvement in performance in the WCST. Nosuch prefrontal activations were seen in schizophrenic patients treatedwith placebo. Similarly, increased prefrontal activations have been notedin schizophrenic patients whilst performing the WCST in the presence ofapomorphine. More recently Dolan et al.32, using PET based rCBFmeasures, have examined the functional effects of dopaminergicmanipulation on the regulation of a network of brain areas activatedduring a verbal fluency task in normal subjects and drug-free schizo-phrenic patients. Under conditions of paced verbal fluency (whereperformance is matched between patients and controls), schizophrenicpatients failed to show the expected anterior cingulate activation seen innormal volunteers (Fig. 2a). However, following apomorphine, 10 meg/kg s.c, the verbal fluency induced deficit of rCBF response in the anteriorcingulate was reversed (Fig. 2b). Thus a cognitive task induced deficit ofanterior cingulate cortex function in schizophrenic patients can bemodulated by manipulation of dopaminergic neurotransmission. Thisstudy and the earlier work of Weinberger provide direct in vivo evidencethat a neurochemical substrate underlying abnormal frontal function inschizophrenia could be dopaminergic neurotransmission. To substantiatethis work, methodological advances are required to enable direct in vivomeasurements of prefrontal dopamineric neurotransmission. Somepossible advances, relevant to this goal, are detailed below and includeattempts to measure endogenous dopamine release with radiotracers and

Britith Mtdical BulUtin 1996,52 (No. 3) 521



quantification of cortical signals from dopaminergic radiotracers usinghigh sensitivity PET cameras and novel methods of data analysis.

Novel approaches using PET and SPECT techniques

Detection of endogenous neurotransmitter release

Endogenous neurotransmitter release is a pivotal event in the neuro-chemical regulation of behaviour. A major limitation in understandingsuch an event has been the inability to directly monitor this phenomenonin the living human brain. Theoretically, endogenous neurotransmitterrelease might be detectable by measuring changes over time in thebinding of a PET/SPECT radiotracer in vivo as a consequence of abehavioural or pharmacological manipulation that releases endogenousneurotransmitter. The assumption here is that endogenous neurotrans-mitter levels, at the receptor site, will vary with the manipulation and, asa consequence, the competitive binding of the PET/SPECT radiotracer atthe receptor site will be significantly altered. Increasing evidence isavailable to support this rationale. Thus, predosing subjects withscopolamine, a muscarinic antagonist that releases dopamine in thestriatum, decreases the subsequent binding of the dopamine D2

antagonist nC-raclopride in the striatum33. Similarly, in baboons,predosing with drugs that decrease striatal dopamine concentrations,such as lorazepam (benzodiazepine agonist) or GVG (suicide inhibitor ofGABA transaminase), increases striatal 1lC-raclopride binding34. Theseeffects are most easily explained by the known functional regulation ofnigrostriatal dopamine release by cholinergic and GABAergic systems.Indeed, in vivo dialysis studies, mainly in rats, have shown that the aboveagents alter endogenous dopamine release in the striatum withpredictable consequences for "C-raclopride binding. Furthermore, usingthe dopamine D2 PET radiotracer nC-raclopride, Volkow et al. haveshown reduced binding following methylphenidate 0.5 mg/kg35 andLaurelle et al?6, using the dopamine D2 SPECT radiotracer 123I-iodobenzamide, have demonstrated decreased binding following amphe-tamine 0.3 mg/kg intravenously. The likely explanation for these effects isthat methylphenidate and amphetamine release dopamine which is thenin competition with radiotracer for binding to the D2 receptor. Such orsimilar techniques have a ready application to studies of schizophrenia totest the hypothesis that regional endogenous dopamine release isdisturbed in this condition.

There is also some evidence that behavioural manipulations mayinfluence the binding of a PET radiotracer via endogenous neurotrans-

522 Brttii/)M«<*co/Bu//«hnl996;52(No.3)



mitter release. Using "C-diprenorphine to label central mu, kappa anddelta opioid receptor binding sites in epileptic patients, selectivedisplacement was detected during serial absence seizures induced byhyperventilation37. Provocation of serial absence seizures was associatedwith increased nC-diprenorphine elimination from the association cortexbut not from the thalamus, basal ganglia or cerebellum. These changeswere detected when compared to control subjects hyperventilating andepileptic patients studied without provocation. Most importantly,simulation experiments showed that significant increases of blood flow(up to 10-fold) could not account for the observed changes. Thus, themost likely explanation is that there is endogenous release of opioids inthe association cortex during absence seizures with consequent displace-ment of nC-diprenorphine bound to opioid receptors.

The above are pioneering studies and this methodology may prove tobe of major importance in biological psychiatry research. Analyticalrefinements will be required particularly in making statistical inferencesas to the significance of any observed radiotracer displacement38, as wellas the exact physiological interpretation of such a displacement.

Low dose radiotracer studies using a multiple organs coincidences

counter

This novel approach uses PET radiotracers but sacrifices tomographicinformation in order to reduce the radiation exposure of the subject toabout 100-fold less than a conventional PET study22. Instead of using aPET camera, a modified whole body counter, consisting of a smallnumber of large sodium iodide scintillation counters, is configured todetect the coincident gamma radiation emitted by decaying PETradioisotopes. For appropriate radioligands, such as "C-diprenorphine(opioid receptors) and "C-flumazenil (central benzodiazepine receptors),determination of a tissue receptor occupancy index, at variousconcentrations of exogenously administered drugs, can be achieved22.Because of the low radiation exposure, this method allows repeatedestimations, within a subject, of receptor occupancy changes with drugtreatment. This would permit comprehensive studies of the exactrelationships between pharmacodynamic effects of a drug (such as sideeffects, response to treatment) and receptor occupancy.

Improvements in scanner technology and data analysis

Two recent developments have considerably improved the potential forPET measurement of receptors/neurochemical pathways. These are the

Britiih Medical Bulktin1996fiHNo. 3) 523



increased sensitivity of PET cameras and the development of automaticimage analysis techniques to produce parametric images. Many new PETcameras are capable of acquiring data in a 3D mode which has allowed a4-fold increase in sensitivity of detection2. In addition, automatic pixel bypixel estimation of receptor parameters39 can be achieved, as has been thecase for rCBF measurements (see for example26), thus removing region ofinterest measurements with their attendant observer bias. The applicationof cluster and spectral analysis analysis techniques40 to define thebehaviour of a radiotracer in vivo is also of great interest.

These developments should allow determination of radiotracer bindingin many brain areas hitherto beyond measurement, such as the brainstem, and will enable weak specific signals, such as radiotracer binding tocortical dopamine receptors, to be more easily detected.

Conclusions

PET and SPECT based methods have exquisite sensitivity in the sub-nanomolar range and although very expensive are increasingly availableas research tools. Using PET and SPECT, the assessment of brainreceptors (Bmax or binding potential) in psychiatric patient groups andcontrols is well established. Although change of receptor number inpsychiatric populations has proved elusive, the techniques have success-fully defined the effects of psychotropic drugs including receptoroccupancy, tissue kinetics and the modulatory effects of drugs oncognitive function. So far, such measurements have been applied mainlyto schizophrenic patients and have generated important findings relevantto the neurochemistry and psychopharmacology of schizophrenia. Thetechnology is advancing rapidly and the developments emphasised in thischapter suggest that further innovative clinical studies are to be expected.

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Psychopharmacology - in vivo neurochemistry and pharmacology